Change type of parameter default values to Option<Value> (#8940)

# Description

Fixes #8939.

# User-Facing Changes

- Parameter default values will now be parsed as constants.
- If the default value is not a constant, a parser error is displayed.

# Tests + Formatting

The [only affected
test](d42c2b2dbc/src/tests/test_engine.rs (L325-L328))
has been updated to reflect the new behavior.

crates/nu-protocol/src/parse_error.rs

@@ -338,6 +338,10 @@ pub enum ParseError {
         #[label = "parameter {0} needs to be '{1}' instead of '{2}'"] Span,
     ),
 
+    #[error("Default values should be constant expressions.")]
+    #[diagnostic(code(nu::parser::non_constant_default_value))]
+    NonConstantDefaultValue(#[label = "expected a constant value"] Span),
+
     #[error("Extra columns.")]
     #[diagnostic(code(nu::parser::extra_columns))]
     ExtraColumns(
@@ -472,6 +476,7 @@ impl ParseError {
             ParseError::IncompleteParser(s) => *s,
             ParseError::RestNeedsName(s) => *s,
             ParseError::ParameterMismatchType(_, _, _, s) => *s,
+            ParseError::NonConstantDefaultValue(s) => *s,
             ParseError::ExtraColumns(_, s) => *s,
             ParseError::MissingColumns(_, s) => *s,
             ParseError::AssignmentMismatch(_, _, s) => *s,

crates/nu-protocol/src/signature.rs

@@ -11,6 +11,7 @@ use crate::PipelineData;
 use crate::ShellError;
 use crate::SyntaxShape;
 use crate::Type;
+use crate::Value;
 use crate::VarId;
 use std::fmt::Write;
 
@@ -35,7 +36,7 @@ pub struct PositionalArg {
 
     // For custom commands
     pub var_id: Option<VarId>,
-    pub default_value: Option<Expression>,
+    pub default_value: Option<Value>,
 }
 
 #[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]